Solid compositions generating hydrogen by combustion, comprising an alkali metal borohydride and an ammonium salt

ABSTRACT

The invention relates to solid compositions that can decompose generating hydrogen in a self-sustaining combustion reaction after this reaction has been initiated by an appropriate heat source. These compositions comprise an alkali metal borohydride and an oxidizing mineral ammonium salt of formula NH 4 Y, in which Y represents a group consisting only of nitrogen and oxygen. The ratio of the weight content of alkali metal borohydride to the weight content of the salt of general formula NH 4 Y is between 1 and 4. Thus, high hydrogen mass yields are obtained, of around 8 to 14%, thereby making it possible to reduce the size and the weight of the hydrogen generators used in certain systems, especially in proton exchange membrane fuel cells.

[0001] The present invention relates to the field of hydrogen generators, hydrogen being a gas widely used as fuel or as reducing agent in many industrial processes or devices.

[0002] More specifically, the subject of the invention is novel solid compositions that can decompose generating hydrogen in a self-sustaining combustion reaction, and the use of these compositions to supply proton exchange membrane fuel cells with hydrogen.

[0003] Many solid compositions generating hydrogen by combustion are known, especially for producing hydrogen intended to serve as fuel in chemical lasers.

[0004] U.S. Pat. No. 3,948,699 describes solid compositions generating hydrogen by combustion, these consisting of a mixture of a metal borohydride, for example sodium borohydride NaBH₄, with a metal oxide, for example iron oxide FE₂O₃ or aluminium oxide Al₂O₃.

[0005] However, the mass yields of hydrogen produced are low, less than 5% expressed as weight of hydrogen obtained with respect to the total weight of the composition.

[0006] U.S. Pat. No. 4,064,225 describes other solid compositions generating hydrogen by combustion, these consisting of a mixture of a metal borohydride, for example sodium borohydride, with ammonium sulphate (NH₄)₂SO₄ or ammonium dichromate (NH₄)₂Cr₂O₇.

[0007] The mass yields are slightly higher, around 6%.

[0008] Such hydrogen yields, less than or about 5%, prove in practice to be insufficient, especially when miniaturization of systems is desired, for example when it is wished to replace the batteries of portable electronic systems, such as telephones and computers, with miniature hydrogen fuel cells.

[0009] A person skilled in the art therefore is permanently trying to find novel solid compositions generating hydrogen by combustion which provide better hydrogen mass yields so as to reduce as far as possible the size and the weight of generators in these portable miniaturized systems.

[0010] The present invention provides a solution to this problem.

[0011] More specifically, the subject of the invention is a novel solid composition that can decompose generating hydrogen in a self-sustaining combustion reaction after this reaction has been initiated by an appropriate heat source, the said composition comprising an alkali metal borohydride and an oxidizing mineral ammonium salt satisfying the general formula NH₄Y in which Y represents a group consisting only of nitrogen and oxygen, the ratio of the weight content of alkali metal borohydride to the weight content of the salt of general formula NH₄Y being between 1 and 4.

[0012] Unexpectedly, it has been found that such compositions result in a hydrogen mass yield of around 8% to 14% depending on the nature and the relative proportions of the constituents, this constituting a particularly advantageous technical and economic advance for the reasons mentioned above.

[0013] Preferably, Y represents the nitrate group (—NO₃) or the dinitramide group

[0014] Ammonium nitrate is particularly preferred.

[0015] According to a preferred embodiment, the compositions according to the invention contain no organic matter, that is to say they only consist of mineral compounds.

[0016] Particularly preferably, they essentially consist of the alkali metal borohydride and the mineral ammonium salt of formula NH₄Y, that is to say these constituents are the predominant ones by weight. It should be understood that the sum of the weight contents of the alkali metal borohydride and of the ammonium salt of formula NH₄Y is greater than or equal to 75%, better still greater than or equal to 90% and even greater than or equal to 95%, with respect to the total weight of the composition.

[0017] Compositions consisting only of an alkali metal borohydride and a salt of formula NH₄Y are particularly preferred. The expression “consisting only” should be understood to mean that the compositions may, however, include the impurities present in the as-received or purified alkali metal borohydride and in the as-received or purified salt of formula NH₄Y which are used, or else additives such as stabilizers, whether these products are commercial products or are synthesized using standard methods.

[0018] When the compositions do not consist only of the alkali metal borohydride and the ammonium salt of formula NH₄Y, they may, for example, also include other metal, especially alkaline-earth metal, borohydrides and/or a metal hydride and/or other oxidizing mineral salts such as alkali metal nitrates, ammonium sulphate, ammonium dichromate, iron oxides and aluminium oxide.

[0019] According to another preferred embodiment of the invention, the alkali metal borohydride is chosen from the group consisting of lithium borohydride, sodium borohydride and mixtures thereof.

[0020] According to another preferred embodiment, the solid compositions according to the invention are in the form of a compact material, having an intrinsic shape, for example, and as a preference, in the form of pellets or particles. The particles may have any, preferably spherical, ovoid or cylindrical, shape.

[0021] The pellets may also have any thickness and any peripheral geometry, for example circular, elliptical, square or rectangular geometry.

[0022] The thickness of the pellets may not be constant.

[0023] The solid compositions according to the invention may be obtained by analogy with the methods described, used to obtain the aforementioned solid compositions of the prior art, for example by simple mixing of the constituents, grinding and then mechanical homogenization. It is also possible to grind the constituents before mixing them, or else to use constituents already in a pulverulent form.

[0024] The compositions may also be obtained by granulation.

[0025] When, preferably, it is desired to obtain a solid composition in the form of a compact material, the homogeneous, granular or pulverulent, mixture of the various constituents may, for example, be agglomerated by compacting them in a pressing container having the desired shape and dimensions of the compact material.

[0026] It is also possible to obtain a compact material by putting the constituents into solution and/or suspension in a liquid medium. After homogenization and injection into a mould having the appropriate dimensions desired for the compact material, the liquid is removed, for example by evaporation, thereby producing a compact material.

[0027] The subject of the present invention is also a process for generating hydrogen by self-sustaining combustion of a solid composition comprising an alkali metal borohydride and an oxidizing mineral ammonium salt.

[0028] According to this process, a homogeneous pulverulent or granular, solid composition, comprising an alkali metal borohydride and an ammonium salt of general formula NH₄Y, Y having the aforementioned meaning and the ratio of the weight content of alkali metal borohydride to the weight content of the salt of general formula NH₄Y being between 1 and 4, is firstly produced.

[0029] Next, this composition is agglomerated using appropriate means, for example those mentioned above, so as to form a compact material, and then the compact material is placed in a combustion chamber which is purged with an inert gas or in which a vacuum is created.

[0030] When the dead volume (the volume remaining in the chamber after the compact material has been placed therein) is low, such a purge may in practice be unnecessary.

[0031] Combustion of the compact material is then initiated using an appropriate heat source, which causes the self-sustaining combustion of the material with generation of hydrogen until the end of combustion.

[0032] Appropriate heat sources for initiating the combustion by the “Joule” effect are well known to those skilled in the art, especially electrical initiators. It is perfectly suitable to use a nickel-chromium ignition filament placed in contact with or encapsulated in the composition to be initiated, on which filament a sufficient voltage and a current of sufficient intensity (and therefore a sufficient power) are imposed. It is possible, for example, for a given voltage to increase the intensity of the current until the combustion is initiated.

[0033] In certain cases, in order to promote ignition, a conventional relay-ignition powder, well known to those skilled in the art, may be placed between the filament and the compact material.

[0034] The subject of the present invention is also a pyrotechnic hydrogen generator intended to supply a proton exchange membrane fuel cell with hydrogen, comprising an aforementioned solid composition according to the invention.

[0035] Fuel cells operating with hydrogen, also called proton exchange membrane fuel cells, are well known to those skilled in the art.

[0036] Such a fuel cell essentially consists of two parts:

[0037] the core of the fuel cell, consisting of one or more electrochemical cells mounted in series, which produces the electrical energy;

[0038] the fuel, namely hydrogen, reservoir.

[0039] Attached to these two main parts are auxiliary systems, especially those for supplying the core of the fuel cell with hydrogen, for removing the water produced or for cooling.

[0040] Each cell of the core of the fuel cell delivers the electrical energy as a result of the two electrochemical reactions taking place at the two electrodes which are immersed in an electrolyte and separated by a proton exchange membrane. In the presence of a catalyst, the hydrogen at the anode is oxidized, separating into protons and electrons. The flux of protons passes through the membrane, while the electrons are captured by an external electrical circuit. The protons and the electrons recombine with oxygen on the other side of the membrane, at the cathode, in order to produce water.

[0041] The pyrotechnic hydrogen generators according to the invention essentially consist of one or more chambers in which a solid composition according to the invention, separate means for initiating the combustion of the composition in each of the chambers, means for actuating this initiation and means for transferring the hydrogen liberated in the chambers to the anode of a cell of the core of the fuel cell are placed.

[0042] Preferably, the overall amount of hydrogen capable of being delivered by the generator is liberated discontinuously by a separate initiation of the combustion of the solid compositions contained in the various chambers. The mass of solid composition in each chamber may be identical or different from one chamber to another. This latter variant makes it possible to liberate hydrogen in an amount tailored to a particular need.

[0043] The various chambers may run into a chamber in which the liberated hydrogen expands, this expansion chamber being connected to the anode compartment of a cell, or one of the walls of which is at least partly formed by the anode.

[0044] The subject of the present invention is also a proton exchange membrane fuel cell using hydrogen as fuel, comprising at least one electrochemical cell and at least one aforementioned pyrotechnic hydrogen generator according to the invention, connected to the anode compartment of the cell.

[0045] The following non-limiting examples illustrate the invention and the advantages that it affords.

EXAMPLE 1

[0046] Solid composition consisting of a mixture of NaBH₄ and NH₄NO₃ in relative weight proportions of 60/40 respectively.

[0047] A mixture of 90 g of NaBH₄ and 60 g of NH₄NO₃ containing 7% by weight of KNO₃ as phase-stabilizing additive were ground and then homogenized.

[0048] Next, one portion of the homogeneous, pulverulent mixture thus obtained was put into and then compacted in the compression die of a pelletizer having the desired pellet geometry, under a pressure of 10⁷ Pa (100 bar).

[0049] Next, the circular pellet thus obtained, having a diameter of 5 mm and a mass of 80 mg, was put into a combustion chamber having a volume of 10 cm³, the said chamber being fitted with a pressure gauge, a temperature probe and a standard ignition device comprising a nickel (80 wt %)-chromium (20 wt %) filament. The pellet was brought into contact with the filament and the chamber then purged with an inert gas (nitrogen) at an absolute pressure of 10⁵ Pa (1 bar) The filament was then heated by the Joule effect until initiation of the combustion of the composition.

[0050] Once initiated, the combustion of the composition was self-sustaining and lasted about 3 s.

[0051] The measured combustion temperature was 1044 K.

[0052] After combustion, the chamber was cooled to room temperature and the pressure in the chamber then noted.

[0053] The measured increase in pressure and the analysis of the gases present after combustion, using chromatography coupled to a mass spectrometer, were used to calculate a hydrogen mass yield of 8.2%, expressed as g of hydrogen liberated per g of solid composition.

EXAMPLES 2 to 5

[0054] Solid compositions consisting of NaBH₄/NH₄NO₃ mixtures in other weight proportions.

[0055] For these examples, the same procedure as in Example 1 was strictly carried out, with the same two constituents (NaBH₄ and NH₄NO₃), with a pellet of the same mass being obtained, but with different weight proportions of these two constituents.

[0056] Table 1 below specifies, for each example, the NaBH₄/NH₄NO₃ weight proportions of the composition, the measured combustion temperature and the hydrogen mass yield obtained. TABLE 1 Combustion Hydrogen NaBH₄/NH₄NO₃ temperature yield mass ratio (K) (%) Example 2 65/35 958 8.7 Example 3 70/30 865 9.0 Example 4 75/25 515 9.3 Example 5 78/22 265 9.4

EXAMPLES 6 to 10

[0057] Solid compositions consisting of LiBH₄/NH₄NO₃ mixtures in various weight proportions.

[0058] For these examples, the same procedure as in Examples 1 to 5 was strictly carried out, but the sodium borohydride was replaced with lithium borohydride.

[0059] Table 2 below specifies, for each example, the LiBH₄/NH₄NO₃ weight proportions of the composition, the measured combustion temperature and the hydrogen mass yield obtained. Combustion Hydrogen LiBH₄/NH₄NO₃ temperature yield mass ratio (K) (%) Example 6 50/50 1500 11.8 Example 7 55/45 1435 12.4 Example 8 60/40 1320 13.1 Example 9 65/35 1060 13.4 Example 10 70/30 805 13.5 

1. Solid composition that can decompose generating hydrogen in a self-sustaining combustion reaction after this reaction has been initiated by an appropriate heat source, the said composition comprising an alkali metal borohydride and an oxidizing mineral ammonium salt, characterized in that the said salt satisfies the general formula NH₄Y in which Y represents a group consisting only of nitrogen and oxygen, and in that the ratio of the weight content of alkali metal borohydride to the weight content of the salt of general formula NH₄Y is between 1 and
 4. 2. Solid composition according to claim 1, characterized in that Y represents the nitrate or dinitramide group.
 3. Solid composition according to claim 1, characterized in that it is in the form of a compact material
 4. Solid composition according to claim 3, characterized in that the compact material is a pellet or a particle.
 5. Solid composition according to claim 1, characterized in that it contains no organic matter.
 6. Solid composition according to claim 1, characterized in that it essentially consists of an alkali metal borohydride and a salt of general formula NH₄Y.
 7. Solid composition according to claim 1, characterized in that the sum of the weight contents of alkali metal borohydride and of the salt of general formula NH₄Y is greater than or equal to 90% compared with the total weight of the composition.
 8. Solid composition according to claim 1, characterized in that the alkali metal borohydride is chosen from the group consisting of lithium borohydride, sodium borohydride and mixtures thereof.
 9. Process for generating hydrogen by self-sustaining combustion of a solid composition comprising an alkali metal borohydride and an oxidizing mineral ammonium salt, characterized in that: a homogenous, pulverulent or granular, solid composition comprising an alkali metal borohydride and an ammonium salt of general formula NH₄Y, in which Y represents a group consisting only of nitrogen and oxygen, the ratio of the weight content of alkali metal borohydride to the weight content of the salt of general formula NH₄Y being between 1 and 4, is produced; next, this composition is agglomerated using appropriate means so as to form a compact material; the compact material is placed in a combustion chamber; and the combustion of the compact material is initiated using an appropriate heat source, which causes the self-sustaining combustion of the material with generation of hydrogen until the end of combustion.
 10. Hydrogen generator intended to supply a proton exchange membrane fuel cell with hydrogen, characterized in that this generator is a pyrotechnic generator comprising a solid composition according to claim
 1. 11. Proton exchange membrane fuel cell using hydrogen as fuel, comprising at least one electrochemical cell and a hydrogen generator connected to the anode compartment of the electrochemical cell, characterized in that this hydrogen generator is a pyrotechnic generator according to claim
 10. 